I had to do a major system restore, which involved a full OS rebuild, and then restore from nightly backups for email. For some reason around 20% of my outlook users aren't receiving any new email when they do a send/receive. All of their mail is showing up in the web client, just not in Outlook. There are no error messages on the Outlook side either.
Deleting the .zdb file doesn't fix it, but deleting the account in Outlook and then recreating it does. Unfortunately some of my users with *very* large mailboxes are overseas, so this isn't an acceptable workaround for them.